Accurate circuit models derived from electromagnetic simulations have been used to fabricate photomixer sources with optimized high-impedance antennas. Output powers on the order of 1 W were measured for various designs spanning 0.6-2.7 THz. The improvement in output power ranged from 3 to 10 dB over more conventionally designed photomixers using broad-band log-spiral antennas. Measured data on single dipoles, twin dipoles, and twin slots are in good agreement with the characteristics predicted by the design simulations.

We study the Curie temperature and hole density of (Ga,Mn)As while systematically varying the As-antisite density. Hole compensation by As-antisites limits the Curie temperature and can completely quench long-range ferromagnetic order in the low doping regime of 1-2% Mn. Samples are grown by molecular beam epitaxy without substrate rotation in order to smoothly vary the As to Ga flux ratio across a single wafer. This technique allows for a systematic study of the effect of As stoichiometry on the structural, electronic, and magnetic properties of (Ga,Mn)As. For concentrations less than 1.5% Mn, a strong deviation from TC ∝ p 0.33 is observed. Our results emphasize that proper control of As-antisite compensation is critical for controlling the Curie temperatures in (Ga,Mn)As at the low doping limit.

A traveling-wave THz photomixer based on a free-space optical-THz phase-matching scheme is proposed. A dc-biased coplanar strip line fabricated on low-temperature-grown GaAs serves as the active area of the device, and is illuminated by two noncollinear laser beams which generate interference fringes that are accompanied by THz waves. The device with the laser-power-handling capability over 300 mW and a 3-dB bandwidth of 1.8 THz was experimentally demonstrated. The results show that traveling-wave photomixers have the potential to surpass small-area designs.

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We report photomixer devices fabricated on a material consisting of self-assembled ErAs islands in GaAs, which is grown by molecular beam epitaxy. The devices perform comparably and provide an alternative to those made from low-temperature-grown GaAs. The photomixer's frequency response demonstrates that the material is a photoconductor with subpicosecond response time, in agreement with time-resolved differential reflectance measurements. The material also provides the other needed properties such as high photocarrier mobility and high breakdown field, which exceeds 2 ϫ10 5 V/cm. The maximum output power before device failure at frequencies of 1 THz was of order 0.1 W. This material has the potential to allow engineering of key photomixer properties such as the response time and dark resistance. © 2000 American Institute of Physics. ͓S0003-6951͑00͒01924-0͔A promising approach for the generation of submillimeter ͑THz͒ waves is optical-heterodyne conversion, or photomixing, by photoconductive materials with response times in the picosecond ͑ps͒ range.1 The response time is a crucial parameter for device design and performance, and engineering control over this time is desirable. Brown et al.2 demonstrated photomixer devices based on low-temperature-grown ͑LTG͒ GaAs, which is the most widely used ultrafast photoconductor. 3-5Here we report on the performance of photomixers made from an alternative material consisting of layers of ErAs islands separated by GaAs, as shown schematically in part ͑a͒ of Fig. 1. The use of this material was motivated by the recent finding that carrier trapping in GaAs:ErAs occurs on a subps time scale, similar to LTG GaAs. 6 We believe that this material is particularly promising because one can engineer its microstructure, which in turn controls the electronic properties. The superlattice period, for example, controls the carrier trapping time.6 With molecular beam epitaxy ͑MBE͒ it is straightforward to control this period and hence to achieve a desired carrier trapping time. Other properties that strongly depend on the microstructure are the dark resistance and the thermal conductivity.The material used for the photomixer devices in the present work is grown by MBE on a semi-insulating ͑100͒ GaAs substrate. 60 periods of 1.2 monolayers of ErAs and 20 nm of GaAs are deposited on the substrate at a temperature of 535°C, resulting in a 1.2 m thick film. The material is nominally identical to one of the samples discussed by Kadow et al., 6 who also present details of the MBE growth. The response time measured by time-resolved differential optical reflectance for this material is 120 fs. The island diameters are between 1 and 2 nm, and the density is about 7ϫ10 12 cm Ϫ2 . Interdigitated electrodes with a finger width of 0.2 m and a finger spacing of 1.8 m were fabricated on the material surface. These electrodes are located at the vertex of a planar self-complementary log-spiral antenna. The interdigitated electrodes and the inner part of the spiral antenna are defined by electron beam l...

A solution phase route to films of nanocrystalline titanium nitride and carbonitride is reported. Transamination of Ti(NMe 2 ) 4 with primary amines results in primary amide groups which can self condense forming alkylimide bridges. The use of controlled amounts of amine results in sols in tetrahydrofuran; these can be coated onto silica substrates via a dip coating method. Pyrolysis of these films under ammonia or nitrogen leads to TiN and Ti(C,N) respectively.

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